Many of the recent efforts in the design of circuits for powering high intensity discharge (HID) lamps have focused on cost reduction by minimizing parts count. One existing approach along those lines is described in FIG. 1. In circuit 100, the conventional full-bridge inverter is replaced by a half-bridge inverter (SW1,SW2). Circuit 100 includes a pair of low-side capacitors (CLOWSIDE1,CLOWSIDE2) which functionally replace the two removed inverter transistors. Furthermore, the conventional buck regulator, which normally serves to regulate the power level of the input voltage (VRAIL) and thereby regulate the power provided to the lamp, is effectively integrated into circuit 100 (rather than being a separate circuit) by driving the inverter switches (SW1,SW2) with a pulse-width-modulation (PWM) sequence, which allows circuit 100 to control both the polarity and the power level of the excitation that is supplied to the lamp.
Although circuit 100 appears to represent a considerable advance in the art, it is known to have several limitations.
A first limitation of circuit 100 is that, because the maximum voltage (i.e., the so-called “open circuit voltage,” or OCV) provided by a conventional half-bridge is, for a given value of input voltage VRAIL, only one-half that which is provided by a conventional full-bridge, circuit 100 requires innovative starting methods, such as resonant or quasi-resonant starting, in order to generate an OCV that is sufficiently high to ensure successful starting of the lamp. Those innovative starting method necessarily require high frequency operation.
A second limitation of circuit 100 stems from the fact that the low-side capacitors CLOWSIDE1,CLOWSIDE2 can only support lamp current flow of a given polarity for a very short time.
As a consequence of both the first and the second aforementioned limitations, circuit 100 does not offer the ability to provide low frequency (e.g., 1 hertz) excitation of the lamp during the starting phase and/or during steady-state powering of the lamp. Some prior designs have shown exemplary performance using a low frequency (e.g., 1 hertz or so) square wave excitation during the starting phase in order to expedite the heating of the lamp electrodes while minimizing possible sputtering and/or loss of lamp conduction. It is believed that, during the starting phase, use of a low frequency or direct current (DC) excitation is to be preferred over use of a high frequency excitation. The reason for this is that commutation (i.e., reversal) of lamp polarity is a troublesome event until both of the lamp electrodes have become hot enough to be effective thermionic emitters, so fewer commutations (i.e., low frequency excitation) during the starting phase is better.
A third limitation of circuit 100 is that it is limited to operating such that the lamp current has a zero average value (i.e., no DC component). For some applications, such as for asymmetric (e.g., vertically oriented) lamps operated at constant power, it is desirable to provide a lamp current having a non-zero average (i.e., DC) value. Circuit 100 is not capable of providing that type of operation.
A fourth limitation of circuit 100 is that the low-side capacitors CLOWSIDE1,CLOWSIDE2 require relatively large value (e.g., 68 microfarads) electrolytic capacitors; such capacitors are costly, large, and prone to reliability problems (especially in the high temperature operating environment that is typically encountered within HID ballasts). An additional negative consequence of large value low-side capacitors is a large inrush current that occurs when AC power is initially applied to the circuit. The series combination of CLOWSIDE1,CLOWSIDE2 typically serves a second function as the so-called bulk capacitors for the front-end circuitry (e.g., a combination of a full-wave rectifier circuit and a power factor correcting DC-to-DC converter, such as boost converter or a buck converter) that provides VRAIL. Upon application of AC power to the circuit, the typical front-end circuitry causes the bulk capacitor(s) to rapidly charge to the peak value of the voltage supplied by the AC power source. This rapid charging results in a potentially large inrush current (which is especially large when AC power is initially applied while the instantaneous value of the voltage of the AC power source is at or near its peak value). Large inrush currents are generally acknowledged to be quite undesirable, and are responsible for a host of difficulties (e.g., problems with wiring, component failures, non-compliance with regulations, need for additional circuitry to limit the inrush current, etc.).
Thus, a need exists for a circuit that preserves many of the advantages of circuit 100, but that also provides for low frequency excitation of the lamp during the starting phase and during steady-state powering of the lamp. A further need exists for a circuit that offers the ability to power the lamp with a non-zero average current. A further need exists for a circuit in which the low-side capacitors can be realized in a more cost-effective and space-efficient manner while enhancing the long-term reliability, and substantially reducing the peak inrush current, of the circuit. A circuit with these attributes would represent a considerable advance over the prior art.